On Saturday, September 26, 2020 at 12:33:40 PM UTC-7 Ian Lance Taylor wrote:
> On Fri, Sep 25, 2020 at 6:29 PM Alex Besogonov <alex.be...@gmail.com>
> wrote:
> > If you mutate the value, then only the current goroutine is affected
> (not even its existing children). Basically, the goroutine-local context is
> copied on new goroutine launch.
> > That's how pprof labels work right now, btw.
> Labels are not variables. Variables have values that can be changed.
>
Most people don't need fully mutable goroutine-specific variables, but
something like an automatic context carrier. It's immutable, so this would
mostly solve the mutation problem.
So in your example you'll have to write:
for _, p := range s { threadCtx =
threadCtx.WithValue(p.SlowMethod(threadCtx.get()));}
This makes it clear that there's causal relationship between the current
value and the next one, so the loop can't be naïvely parallelized. Of
course, a very determined programmer can still shoot themselves in the foot
(by reading and replacing the automatic context inside p.SlowMethod), but
the footgun here is pretty explicit.
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